The regulation of prothrombin activation in vitro is being investigated by kinethic techniques. The quantitative contributions of the various accessory factors to Factor Xa, i.e. Factor VA, Ca 2 ion and phospholipid, in catalyzing prothrombin conversion to thrombin over a wide range of concentrations of these species is being determined. The reaction time course data are analyzed by fitting the data to curves derived from mathematical models for the activation process using digital computer simulation and least squares fitting procedures.